将String改为StringBuilder，程序就会出错？

Question

目标：打印出给定字符串的所有的最长公共子序列。

示例：第一行：acb，第二行abc；输出结果：ac，ab

正确程序：

import java.util.HashSet;
import java.util.Scanner;
import java.util.Set;
public class LCS {
    private static int[][] dp;
    private static String s1, s2;
    private final static Set<String> set = new HashSet<>();
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        s1 = in.nextLine(); int len1 = s1.length();
        s2 = in.nextLine(); int len2 = s2.length();
        //dp结果计算
        dp = new int[len1+1][len2+1];
        for (int i = 1; i < len1+1; i++) {
            for (int j = 1; j < len2+1; j++) {
                if (s1.charAt(i-1) == s2.charAt(j-1)) {
                    dp[i][j] = dp[i-1][j-1] + 1;
                } else {
                    dp[i][j] = Math.max(dp[i-1][j], dp[i][j-1]);
                }
            }
        }
        //打印dp结果
        for (int i = 0; i < len1+1; i++) {
            for (int j = 0; j < len2+1; j++) {
                System.out.print(dp[i][j] + " ");
            }
            System.out.println();
        }
        //输出所有的LCS
        String res = "";
        traceBack(len1, len2, res);
        System.out.println(set.toString());
    }

    private static void traceBack(int len1, int len2, String res) {
        while (len1 > 0 && len2 > 0) {
            if (s1.charAt(len1-1) == s2.charAt(len2-1)) {
                res += s1.charAt(len1-1);
                //res.append(s1.charAt(len1-1));
                len1--; len2--;
            } else {
                if (dp[len1][len2-1] > dp[len1-1][len2]) {
                    len2--;
                } else if (dp[len1][len2-1] < dp[len1-1][len2]) {
                    len1--;
                } else {
                    traceBack(len1-1, len2, res);
                    traceBack(len1, len2-1, res);
                    return;
                }
            }
        }
        set.add(new StringBuilder(res).reverse().toString());
    }
}

错误代码，求大佬指点：

示例：第一行：acb，第二行abc；输出结果：ac，abac，求解哪里出问题了？谢谢

import java.util.HashSet;
import java.util.Scanner;
import java.util.Set;

/**
 * 问题：利用StringBuilder代替String就会有问题
 */
public class LCS {
    private static int[][] dp;
    private static String s1, s2;
    private final static Set<String> set = new HashSet<>();
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        s1 = in.nextLine(); int len1 = s1.length();
        s2 = in.nextLine(); int len2 = s2.length();
        //dp结果计算
        dp = new int[len1+1][len2+1];
        for (int i = 1; i < len1+1; i++) {
            for (int j = 1; j < len2+1; j++) {
                if (s1.charAt(i-1) == s2.charAt(j-1)) {
                    dp[i][j] = dp[i-1][j-1] + 1;
                } else {
                    dp[i][j] = Math.max(dp[i-1][j], dp[i][j-1]);
                }
            }
        }
        for (int i = 0; i < len1+1; i++) {
            for (int j = 0; j < len2+1; j++) {
                System.out.print(dp[i][j] + " ");
            }
            System.out.println();
        }
        //输出所有的LCS
        StringBuilder res = new StringBuilder();
        traceBack(len1, len2, res);
        System.out.println(set.toString());
    }

    private static void traceBack(int len1, int len2, StringBuilder res) {
        while (len1 > 0 && len2 > 0) {
            if (s1.charAt(len1-1) == s2.charAt(len2-1)) {
                res.append(s1.charAt(len1-1));
                len1--; len2--;
            } else {
                if (dp[len1][len2-1] > dp[len1-1][len2]) {
                    len2--;
                } else if (dp[len1][len2-1] < dp[len1-1][len2]) {
                    len1--;
                } else {
                    traceBack(len1-1, len2, res);
                    traceBack(len1, len2-1, res);
                    return;
                }
            }
        }
        set.add(new StringBuilder(res).reverse().toString());
    }
}

Answer 1

traceBack(len1-1, len2, res);
traceBack(len1, len2-1, res);

执行第一行后res会发生改变。